# HG changeset patch
# User mdoerr
# Date 1405585291 -7200
# Node ID 63e0c47ca943c9ff10441e084ad302060bc1bba5
# Parent  6ad19ab9417663fca11049b4ede256be61ba2ba2
8050972: Concurrency problem in PcDesc cache
Summary: The entries of the PcDesc cache in nmethods are not declared as volatile, but they are accessed and modified by several threads concurrently.
Reviewed-by: kvn, dholmes, dcubed

diff -r 6ad19ab94176 -r 63e0c47ca943 src/share/vm/code/nmethod.hpp
--- a/src/share/vm/code/nmethod.hpp	Tue Jul 22 07:35:52 2014 -0700
+++ b/src/share/vm/code/nmethod.hpp	Thu Jul 17 10:21:31 2014 +0200
@@ -69,7 +69,12 @@
   friend class VMStructs;
  private:
   enum { cache_size = 4 };
-  PcDesc* _pc_descs[cache_size]; // last cache_size pc_descs found
+  // The array elements MUST be volatile! Several threads may modify
+  // and read from the cache concurrently. find_pc_desc_internal has
+  // returned wrong results. C++ compiler (namely xlC12) may duplicate
+  // C++ field accesses if the elements are not volatile.
+  typedef PcDesc* PcDescPtr;
+  volatile PcDescPtr _pc_descs[cache_size]; // last cache_size pc_descs found
  public:
   PcDescCache() { debug_only(_pc_descs[0] = NULL); }
   void    reset_to(PcDesc* initial_pc_desc);